The membrane-bound enzyme methane monooxygenase (pMMO) is a mixed function oxygenase that initiates the metabolism of methane in methanotrophic bacteria. Although it is shown that copper is an important factor in pMMO expression and activity, its role and binding structure are still not clear. In recent studies, the whole cells and cell-free extracts from 13 or 8 were investigated by ESR spectroscopy. All of these samples exhibit the copper-binding site with g = 2.24 and A = 80-188 G. Under oxidized state, a new ESR signal with g = 2.13 was detected as type 2 signal decreased. As hypothesized by others for pMMO in Bath, the possible explanation for this new signal is that a copper cluster existed in pMMO either as a binuclear copper center or a trinuclear copper center needs further study.